1. Field of the Invention
The present invention relates to a control device for a vibration type actuator.
2. Related Background Art
In general, a vibration type actuator such as an ultrasonic motor includes a vibration member in which an electromechanical energy conversion element such as a piezoelectric element is fixed to an elastic member such as a metal, and a contact member that comes in pressure contact with the vibration member. A control circuit of the vibration type actuator supplies a drive signal to the electro-mechanical energy conversion element to generate a drive vibration on the surface of the vibration member, thereby relatively moving the vibration member and the contact member.
Then, the vibration member includes a piezoelectric element having drive phases at positions having a phase difference of, for example, 90° to each other spatially. The control circuit supplies two-phase alternating signals having a phase difference of 90° to each other temporally to those two drive phases, to generate a traveling wave on the surface of the elastic member and drive the contact member that is in pressure contact with the surface of the elastic member by a frictional force.
A frictional material is formed, adhered or coated on a contact portion between the vibration member and the contact member in order to obtain an appropriate frictional force.
Also, as parameters for controlling the drive speed of the vibration type actuator, there are used the voltage of the drive signal, the frequency of the drive signal, and the phase difference of the drive signals. Among them, a control using the frequency is preferably used because it can obtain both of a broad dynamic range and resolution, independently, and is also excellent in matching with a digital circuit that has been recently developed.
As a conventional frequency control method of the drive signal, the following methods are proposed.
As a first method, there is a method in which the vibration state of the vibration member is outputted, and the output signal is compared with a threshold voltage by a comparator to detect whether the output signal is larger or smaller, to thereby control the vibration type actuator in accordance with the detection, as disclosed in U.S. Pat. No. 5,165,047.
As a second method, there is a method in which the vibration amplitude of the vibration member and the amplitude of an applied voltage supplied to the vibration member are detected, an alternating signal which is detection information is compared with a plurality of voltages which are out of the center of the signal so as to be converted into a plurality of pulse signals, and the amplitude is obtained by using a data conversion table from the pulse width of the pulse waveform. The control method using the plurality of pulse signals is classified into a method in which a plurality of widths are summed to obtain a mean value, and a method in which the amplitude of a triangular wave is obtained from a difference in the pulse width assuming that the amplitude waveform is the triangular wave.
Then, in the above-mentioned first and second control methods, the obtained amplitude is so controlled as to become a given amplitude, and a drive frequency or the like is so controlled as to stabilize the drive speed and the output torque of the vibration type actuator.
However, in the above-mentioned first control method, because it is only possible to judge whether the output signal is larger or smaller than the given level by the result of the comparator, it is necessary to, for example, prepare a plurality of threshold values to detect the amplitude in order to accurately know the vibration amplitude, and a very large number of comparators are thus required.
Also, in the above-mentioned second control method, because a relationship between the pulse width and the amplitude is nonlinear, nonlinear calculation or the data conversion table is required. Also, the method in which the level of the comparison voltage is finely set to obtain the mean value is not enhanced in precision when the comparison level is small. Also, with the method in which the amplitude is calculated assuming that the amplitude waveform is triangular waveform, the precision in the detection of the vibration waveform amplitude of a sine wave is deteriorated.
Accordingly, in order to supply a control device of the vibration type actuator which can detect the drive state of the vibration member at a high speed and simply and cope with the high-speed change of the drive state, more improvements may be required.